Traditionally, electric motors have been used in stationary applications. Because of this, the volume or size of electric motors has not been critical, and sufficient cooling of the electric motors could be achieved by using air, with or without fins, and large motor housings, serving as heat-sinks. More recently, electric motors have been used in mobile applications, such as on automobiles or work machines. Because mobile applications put a premium on smaller size and lower weight, these motors are more power dense and therefore more difficult to cool than their stationary counterparts.
As the size of the motors is decreased, air cooling alone is often insufficient to maintain temperatures of the motors at acceptable levels. Furthermore, because the motors are now often on mobile vehicles, they are subject to a variety of temperature ranges and environments. The air may be dusty or dirty, or the motors may become caked in mud, reducing the ability to air cool the motors. In order to maintain cooling consistency in different environments, electric motors have been developed using liquid cooling.
Initial designs for liquid cooled electric motors included forming fluid passages through the motor housings during casting. In use, as the housings draw heat from motor components in the housing, the heat was drawn away from the housing by the liquids forced through the housing passages. However, casting such passages in housings is difficult and expensive.
One attempt to solve the heating problem is shown in U.S. Pat. No. 5,931,757 to Schmidt. Schmidt discloses an electromechanical transmission for receiving power from an engine. It includes in its outer surface, a single annular channel where oil may accumulate next to a stator in an electric motor system. Although easier to manufacture than cast interior passages, such a single channel may not provide sufficient cooling for the electric motor, and may provide inconsistent heat zones in the stator.
The present invention is directed to overcoming one or more of the deficiencies in the prior art.